The present invention is directed to a multistage vacuum pump and more specifically to a unique valve arrangement providing for a reduction of starting torque, a high pressure idling arrangement for one of the stages during startup and a cylinder liner providing an improved air intake arrangement and high wear resistance.
Australian Patents Nos. 481072 and 516210 and International Patent Application No. PCT/AU82/00128, which are all assigned to the same assignee as the present application, disclose various forms of a reciprocatory piston and cylinder machine having a differential piston and two working spaces. In the practical application of such a machine it is usual to provide multiple cylinders as respective stages of a multistage pump. The machine is particularly well suited for use as a mechanical vacuum pump utilizing solid sealing rings or sleeves in lieu of oil or other liquid lubricant. A four cylinder pump having a pair of parallel coupled high vacuum cylinders, jointly connected in series with a medium vacuum cylinder and a low vacuum cylinder is particularly appropriate and has the advantage of being suitable for construction in well balanced configurations. In prior pumps the connections between these stages have been made by covered passages and external conduits, but these are not readily translated into an internal porting and ducting arrangement, especially because of the presence of two working spaces per working cylinder.
The U.S. Pat. No. 4,560,327, to Bez et al., discloses a porting and ducting arrangement for a pair of adjacent cylinders of a multistage vacuum pump wherein a plurality of passages extend longitudinally in the walls of the cylinders and communicate with the interiors of the cylinders through respective ports. A plurality of recesses in the form of arcuate depressions may be located in the ends of the cylinder walls or in the bottom surface of the cylinder head which register with respective passages or groups of passages and suitable openings are provided in the cylinder head in communication with the recesses for supplying or exhausting fluid to or from the interiors of the cylinders. This patent is also assigned to the assignee of the present application.
Copending U.S. patent application Ser. No. 820,585, now U.S. Pat. No. 4,790,726, filed Jan. 21, 1986, which is a continuation of application Ser. No. 491,967, now abandoned filed Apr. 13, 1983, in the name of Balkau et al., and assigned to the same assignee as the present application, is also directed to a reciprocatory piston and cylinder machine adapted to be used as an oil free vacuum pump. The vacuum pump disclosed in this application is directed to a cylinder having a first portion closed at one end and a second portion contiguous with, but of smaller diameter than, the first portion, and a piston having a cylindrical head portion slidable in the first cylinder portion and a second cylindrical piston portion slidable in the second cylinder portion with said piston head portion having a front face facing the closed cylinder end and an annular back face. A gas inlet is provided for introducing gas to the interior of the first cylinder portion between the front face of the piston head portion and the closed cylinder end on reciprocation of the piston. A first exhaust port is provided for exhausting gas from the interior of the first cylinder portion ahead of the piston head portion by the pumping action of the front face of the piston head portion, a one way valve is provided in the first exhaust port which is operable to permit the exhaust of gas from the interior of the first cylinder portion ahead of the piston head portion and a second exhaust port is provided for the exhaust of gas from the interior of the first cylinder portion behind the piston head portion by the pumping action of the back face of the piston head portion. Sealing means are provided for the piston head portion which includes a sleeve of a low friction material disposed on the cylindrical surface of the piston such that over the temperature range encountered during the normal operation of the pump a mean gap is sustained between the sleeve and the cylinder, which gap is of a maximum size at which leakage of gas past the sleeve is at a level for an acceptable degree of vacuum to be sustained by the pump. A similar sleeve is provided on the second piston portion and resilient means are provided adjacent the end of the sleeve remote from the first piston portion for forcing the sleeve into sliding engagement with the wall of the cylinder. Furthermore the one way valve in the exhaust port is provided with projecting means which are adapted to be engaged by the piston for opening the valve in the exhaust port controlled thereby on each stroke of the piston even though the pressure within the cylinder is too low to open the valve against the force of the spring biasing the valve into normally closed position.